ON THE ANNUAL CYCLE OF UPPER-OCEAN CIRCULATION IN THE EASTERN EQUATORIAL PACIFIC

An oceanic general circulation model is used to investigate the annual cycle of the near-surface currents in the eastern equatorial Pacific Ocean; in particular, the causes of the springtime increase bf eastwar d momentum that reverses the westward surface flow and intensifies the Equatorial Undercurrent are examined. A set of process experiments ar e carried out that isolates effects due to three forcing mechanisms: l ocal zonal and meridional winds, and remote zonal winds. It is demonst rated that the springtime weakening of the local easterly trades is th e primary cause of the eastward-momentum increase. In addition, due to meridional advection, the local southerly wind drives a westward curr ent on the equator throughout the year; this flow is weakest in the sp ring, and therefore this process also contributes to the anomalous eas tward flow. On the other hand, remote forcing tends to weaken the spri ngtime momentum increase: Anomalous easterlies in the far-western and central Pacific during the winter excite upwelling-favorable Kelvin wa ves that generate anomalous westward flow in the eastern Pacific durin g the early spring.